The present invention relates to an electrophotographic image forming apparatus for forming a latent image electrostatically on a charged image carrier by irradiating it with a light beam, developing the latent image, transferring the developed image to a recording medium and then fixing the image on the medium. More particularly, the present invention is concerned with an image forming apparatus for forming dots or pixels having a basic diameter and dots or pixels having diameters greater than the basic diameter by using a single light beam, and forming a high resolution image with tones by such dots.
In an electrophotographic copier, facsimile transceiver, laser beam printer or similar electrophotographic image forming apparatus, use is made of a semiconductor laser for digitally reproducing the tones of an image. Specifically, a laser beam issuing from a semiconductor laser is easy to modulate with respect to pulse width. It is a common practice, therefore, to form dots having desired diameters and areas and thereby a high resolution image with tones by subjecting the laser beam to pulse width modulation. Such a pulse width modulation scheme is suitable for, among others, the reproduction of tones of a color image.
An image forming apparatus of the type described has an image carrier implemented as a photoconductive element. Generally, the sensitivity and residual potential of a photoconductive element tends to vary with the ambient conditions such as temperature and humidity and due to aging. Likewise, a developer, or toner, for developing a latent image formed on the photoconductive element has the amount of charge and other similar factors thereof varied due to the varying ambient conditions and aging. It is therefore difficult to produce a high resolution image with tones stably over a long period of time. Further, when an image whose density is 600 dots per inch is to be formed by dots whose diameter is smaller than about 42 microns, it is difficult to achieve a high resolution image since the toner does not always deposit on the latent image in proportion to the area exposed by the laser beam. This, of course, depends on the particle size of the toner (about 10 microns) as well. Another factor that degrades the tones of a reproduction and particular to the pulse width modulation system is banding. Specifically, when the distance between nearby dots, i.e., the pulse interval of the fundamental clock is reduced to enhance the resolution of an image with tones, banding is likely to occur due to irregular writing in the subscanning direction.